1. Field of the Invention
The present invention relates to a sealed cell, and more specifically, to the improvement of a sealing body with a safety valve in the sealed cell.
2. Background Art
Non-aqueous electrolyte secondary cells are widely used as the driving power sources of portable devices and electric tools because of their high energy density and high capacity.
These cells using flammable organic solvents are required to ensure safety, and for this reason, the sealing body for sealing such a cell includes a current breaking mechanism and a gas releasing mechanism, which operates when the cell internal pressure increases.
FIG. 1 shows a sealed cell having a sealing body including a current breaking mechanism. The sealing body 10 includes a terminal cap 5, a safety valve 3 disposed on the inner surface of the terminal cap 5, a terminal plate 1 disposed on the inner surface of the safety valve 3, and an insulating member 2 providing isolation and insulation between the safety valve 3 and the terminal plate 1. In order to keep the conductive contact between the cap terminal 5 and the safety valve 3, pin-like projections 3c provided on a periphery portion 3b of the safety valve are inserted into counterbored holes 5c formed in a flange portion 5b of the terminal cap. Then, the terminal cap and the safety valve are riveted and fixed. In addition, a convex is formed toward the inside of the cells near the center of the safety valve 3, and an area near the top of the convex (a conductive contact portion) is welded to the terminal plate 1. A break groove, a thin portion of the terminal plate 1, is formed on the terminal plate 1 at the periphery of the area welded to the conductive contact portion.
The current breaking mechanism of the sealed cell operates as follows. When the cell internal pressure increases, the safety valve 3 is pushed up toward the outside of the cell. This results in the breakage of the break groove of the terminal plate 1, which is connected to a conductive contact portion 3a of the safety valve 3, thereby interrupting current supply to the terminal cap 5.
In such a current breaking mechanism, the safety valve is required to be made of a material susceptible to deformation so that the above-described operation can be performed smoothly. The terminal cap, on the other hand, is required to be made of a material having a certain strength because it is exposed to the external environment. To satisfy these requirements, the safety valve is made of a flexible aluminum-based material, and the terminal cap is made of a rigid iron-based material.
In non-aqueous electrolyte secondary cells that are designed to provide large current discharge performance in electric tools and similar devices, the cell temperature may be heated up to 80° C. or more during discharge. When the cells are repeatedly exposed to high temperatures during long-term use, resin components such as the insulating member 2 and a gasket 30 become less flexible. This reduces the contact between the terminal cap 5 and the safety valve 3 in the vicinity of the resin components. As a result, the conduction therebetween becomes unstable, thereby tending to increase the internal resistance of the cell. For this reason, it is desired to firmly join the terminal cap 5 and the safety valve 3.
Well-known techniques on the sealing body include the following Patent Document 1.    Patent Document 1: Japanese Patent Examined Publication No. H05-74904
Patent Document 1 discloses a sealing body including the following cell cap and metal plate. The cell cap includes a cylindrical portion and a flange that is formed on the outer periphery of the cylindrical portion and has a plurality of holes. The metal plate includes a plurality of projections whose height is larger than the thickness of the cell cap. The projections of the metal plate are inserted into the holes of the cell cap, and each part of the projections that protrudes from the holes is pressure-welded. Thereby, the cell cap and the metal plate are fixed.
According to this technique, a sealing body that excels in anchorage strength and assembly accuracy. However, this technology does not review that the safety valve is apt to deform, and thus does not provide a sufficient conductivity of the sealing body.